1. Field of the Invention
The present invention relates to a reflection-type display device and a method for producing the same.
2. Description of the Related Art
Liquid crystal display device (hereinafter “LCDs”) of a reflection type, which perform display by utilizing ambient light as a light source, have been known. Since reflection-type LCDs do not require a backlight as do transmission-type LCDs, reflection-type LCDs are suitably used for various devices which must have a light weight and a thin thickness. In particular, reflection-type LCDs of an active matrix driving type, in which a switching element is provided corresponding to each pixel, are capable of performing display with a high resolution and high quality.
In order to further improve the display performance of a reflection-type LCD, a retroreflection-type LCD which includes a retroreflection plate as a reflective layer for reflecting ambient light is proposed. As used herein, a “retroreflection plate” is an optical element which reflects an incoming ray of light with a plurality of reflection surfaces, regardless of the orientation of the ray, in the direction in which the ray entered the device. For example, a retroreflection plate is composed of a two-dimensional array of minute unit features.
Since retroreflection-type LCDs do not require any polarizing plates, there is no decrease in the efficiency of light utilization associated with the use of polarizing plates, and hence retroreflection-type LCDs can perform brighter display. Moreover, retroreflection-type LCDs are considered as promising because of their potential ability to realize an improved display contrast ratio.
Hereinafter, the structure of a retroreflection-type LCD of an active matrix driving type will be described with reference to the accompanying drawings. FIG. 1A is a schematic cross-sectional view showing a retroreflection-type LCD. FIG. 1B is a plan view showing reflection electrodes in the display device of FIG. 1A. A structure as shown in FIGS. 1A and 1B is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-195788.
As shown in FIG. 1A, the retroreflection-type LCD comprises: a front substrate 110 on which color filters 119, a transparent counter electrode 111, and an alignment film 112 are provided; a rear substrate 109 provided so as to oppose the front substrate 110; and a liquid crystal layer 113 interposed between the substrates 110 and 109. The rear substrate 109 includes: a TFT substrate 101 having a plurality of switching elements (TFTs) formed thereon; an insulating layer 102 which is provided on the TFT substrate 101 and has a surface configuration that exhibits a retroreflection property; a plurality of reflection electrodes 105; and an alignment layer 118. The reflection electrodes 105, which are formed on the insulating layer 102, present an uneven surface corresponding to the surface configuration of the insulating layer 102. As shown in FIG. 1B, the reflection electrodes 105 are formed so as to be spaced apart from one another, corresponding to pixels (which define units of image displaying). Each reflection electrode 105 is connected to a drain electrode 103 of a corresponding switching element on the TFT substrate 101, via a contact hole 104 which is formed in the insulating layer 102. The alignment layer 118, which is formed above the insulating layer 102 and the reflection electrodes 105, has protrusions and depressions corresponding to the surface configuration of the insulating layer 102. The liquid crystal layer 113 may be composed of, for example, a scattering type liquid crystal material which is capable of switching between a light transmitting state and a light scattering state (forward scattering) in accordance with a varying voltage which is applied between the counter electrode 111 and each reflection electrode 105.
In a display device of this structure, the plurality of reflection electrodes 105 function as pixel electrodes and as a retroreflective layer. Hereinafter, the operation of this display device will be described.
While the liquid crystal layer 113 is controlled to be in a transmitting state, light from a light source which lies external to the display device or ambient light is transmitted through the front substrate 110 and the liquid crystal layer 113, and thereafter reflected by the reflection electrodes 105 in the direction in which the light has entered. From the display device under this condition, an image of the eye(s) of the viewer himself or herself is perceived by the viewer, whereby a “black” displaying state is obtained.
On the other hand, while the liquid crystal layer 113 is controlled to be in a scattering state, the light from a light source or ambient light which has been transmitted through the front substrate 110 is scattered in the liquid crystal layer 113. In the case where the liquid crystal layer 113 is a forward scattering-type liquid crystal layer, the scattered light is reflected by the reflective layer 105, further travels through the liquid crystal layer 113 (which is in a scattering state), and goes out in the viewing direction. Since the retroreflection property of the reflective layer 105 is counteracted by the scattering in the liquid crystal layer 113, the incident light does not go back in its incident direction. As a result, a “white” displaying state is obtained.
The retroreflection-type display device described above has the following problems.
Firstly, the reflection electrodes 105 of the shape as shown in FIG. 1B are typically formed by patterning a concavo-convex metal film. However, it is difficult to perform a highly precise patterning for a concavo-convex metal film. In particular, when a metal film having a complicated surface configuration, e.g., a metal film having a corner cube array structure, it is difficult to form the reflection electrodes 105 in the designed manner, because the metal film is likely to be etched with dependence on the size and/or array pattern of the corner cubes. Therefore, it is difficult to reduce the interspaces between adjoining reflection electrodes 105, and thus the aperture ratio is decreased. Furthermore, since any light entering the gaps between reflection electrodes 105 is not reflected, the efficiency of light utilization is lowered, too. In answer to this problem, the Applicant has proposed in Japanese Laid-Open Patent Publication No. 2003-195788, supra, to perform a patterning of the reflection electrodes 105 in a manner to match the array pattern of the corner cubes, thus to reduce the interspaces between adjoining reflection electrodes 10; however, as the size of each corner cube becomes more minuscule, it becomes more difficult to apply this technique. In addition, design freedom of the reflection electrodes 105 is limited.
Secondly, the protrusions and depressions on the surface profile of the alignment film 118 of the rear substrate 109 make it difficult to realize a stable alignment in the liquid crystal layer 113. In the case where there is a large difference in levels between the depressions and the protrusions on the surface of the alignment film 118, the consequent variations in the thickness of the liquid crystal layer 113 may affect the displaying characteristics.
Although a retroreflection-type LCD is exemplified above, those reflection-type LCDs whose reflective layer is a reflection plate having diffuse reflection characteristics also have reflection electrodes which function both as a reflective layer and pixel electrodes, similarly to the structure shown in FIG. 1; therefore, such reflection-type LCDs also have problems similar to those described above.
Such a diffuse reflection-type LCD structure is disclosed in Japanese Patent No. 3166377, for example. In the structure disclosed in Japanese Patent No. 3166377, an insulative film having a large refractive index is formed between a liquid crystal layer and reflection electrodes whose surfaces have protrusions and depressions. As a result, the protrusions and depressions of the reflection electrodes are to some extent planarized by the insulative film, whereby the aforementioned second problem can be improved. However, a voltage drop is caused by the insulative film which is formed between the liquid crystal layer and the reflection electrodes, and therefore a high driving voltage is required.
As described above, in conventional reflection-type LCDS, a metal film whose surface has protrusions and depressions must be patterned in such a manner as to form reflection electrodes which are spaced apart from one another pixel corresponding to the pixels. However, it is difficult to perform a highly precise patterning for such a metal film. Moreover, there is a problem in the case of performing a patterning utilizing protrusions and depressions, in that the design freedom of the reflection electrodes is limited. Thus, it is difficult to enhance the aperture ratio and/or efficiency of light utilization by reducing the interspaces between reflection electrodes. Furthermore, there is also a problem in that, since a liquid crystal layer is formed on a surface having protrusions and depressions, it is difficult to stably control the alignment in the liquid crystal layer.